Physics of driven plasma rotation

A driven plasma rotation is established in plasmas due to the formation of radial electric fields (in the presence of an externally applied magnetic field, B) in the plasma. In instances of strong plasma rotation (velocities v > 10-20 km/s), there can be substantial changes in the plasma parameters as well as the formation of instabilities.

In fusion energy experiments, very localized regions of highly flowing plasmas are often found near the outer edge of the plasma. These flows can be established by heating the plasma edge or by using electrodes to bias the plasma edge.

In space plasmas, regions of strong flows are observed near the earth in the ionosphere and lower magnetosphere region.


Geometry for plasma rotation

In the current ALEXIS experiments, a spatially varying electric field is created in the plasma through the use of a set of concentric rings at one end of the device. Each ring can be independently biased to create a spatially structured (in the radial direction) electric field. A schematic of the ring electrodes is shown below.

The above description greatly simplifies the mechanisms of plasma rotation. In fact, there are a number of mechanisms that can affect the rotation speed of the plasma. These include the plasma viscosity, ion drag, neutral drag, and the electric field shear. Most notably the role the electric field shear and its effects on the level of plasma fluctuations is not yet well understood.